Small Intestine – Structure and Function

Small Intestine – Structure and Function

The small intestine is a vital part of the digestive system, connecting the stomach to the large intestine. It plays a crucial role in the digestion and absorption of nutrients. This article provides an overview of the small intestine’s structure, location, blood supply, and function.

Location:

• The small intestine is located within the abdominal cavity, extending from the left hypochondrium to the right iliac fossa.

• It is divided into three parts:

• Proximal 1/3: Located in the left hypochondrium.

• Middle 1/3: Located in the central part of the abdomen.

• Distal 1/3: Located in the iliac fossa and right pelvic region.

Related Structures:

• Superior: Transverse colon and transverse mesocolon.

• Inferior: Pelvic viscera (rectum, bladder, reproductive organs).

• Right: Cecum and ascending colon.

• Left: Descending colon.

• Anterior: Anterior abdominal wall via the greater omentum.

Differentiating Jejunum and Ileum:

• Diameter: Jejunum is larger than ileum.

• Wall: Jejunum is thicker, with more blood vessels and taller plicae circulares than ileum.

• Lymphoid tissue: Jejunum has lymph nodules, while ileum has Peyer’s patches.

• Location: The jejunal loop usually lies on the left side, while the ileal loop lies on the right.

Structure:

The small intestine is composed of five layers:

1. Mucosa: Contains plicae circulares, villi, lamina propria, lymph, and muscularis mucosae.

2. Submucosa: Contains blood vessels and nerve plexuses.

3. Muscularis: Composed of an outer longitudinal muscle layer and an inner circular muscle layer.

4. Subserosa:

5. Serosa: Peritoneum layer, where the mesentery attaches, lacking this layer.

Appendix:

• A vestigial remnant of the vitelline duct.

• The tip of the appendix can be free or attached to the abdominal wall around the umbilicus by a fibrous band.

Mesentery:

• A fold of peritoneum that connects the small intestinal loops to the posterior abdominal wall.

• Function: Suspends and provides nourishment to the small intestine.

Root of Mesentery:

• The point where the mesentery attaches to the posterior abdominal wall.

• It runs diagonally from the left T12 vertebra to the right L3-4 vertebra, then laterally towards the right sacroiliac joint.

Structures Passing Through the Root of Mesentery:

• Duodenum, abdominal aorta, inferior vena cava, psoas major muscle, right ureter, right gonadal artery.

Mesenteric Border:

• The point where the mesentery attaches to the jejunum and ileum.

Mesentery Structure:

• Consists of two peritoneum layers pressed together, containing:

• Branches of the superior mesenteric artery and vein.

• Lymphatic vessels and lymph nodes.

• Nerve branches from the mesenteric plexus.

Superior Mesenteric Artery:

• Supplies blood to a portion of the pancreas, the entire small intestine, and the large intestine.

Origin of the Superior Mesenteric Artery:

• Arises from the abdominal aorta, at the level of T12 vertebra.

Four Segments of the Superior Mesenteric Artery:

1. Retro-pancreatic segment.

2. Supra and pre-duodenal segment.

3. Segment within the root of the mesentery.

4. Segment within the mesentery.

Lateral Branches of the Superior Mesenteric Artery:

• Inferior pancreaticoduodenal artery.

• Jejunal and ileal arteries.

• Ileocolic artery.

• Right colic artery.

• Middle colic artery.

Distribution of Arcuate Arteries Along Intestinal Loops:

• Proximal intestinal loops: Only one arcuate artery, long and large straight artery.

• Middle intestinal loops: 1-5 arcuate arteries.

• Distal intestinal loops: Fewer arcuate arteries, short and thin.

Five Branches of the Ileocolic Artery:

• Ascending branch.

• Anterior cecal artery.

• Posterior cecal artery.

• Appendicular artery.

• Ileal artery.

Anastomoses of the Superior Mesenteric Artery:

• Anastomosis with the celiac artery: via inferior pancreaticoduodenal artery connecting to the gastroduodenal artery.

• Anastomosis with the inferior mesenteric artery: via the middle and left colic arteries.

Function:

• Digestion and absorption of nutrients from food.

• Movement of food from the stomach to the large intestine.

• Production of digestive hormones.

• Protection of the body from pathogenic bacteria.

The small intestine is a vital organ in the digestive system, playing an essential role in maintaining the body’s health.